It is known that errors exist in a measurement made by a Vector Network Analyzer (VNA). These measurement errors contribute to the uncertainty of the result given by the measurement of a device under test (“DUT”). The measurement errors may be categorized into two types, random errors and systematic errors. Random errors are non-repeatable variations of the same measurement due to physical and environmental changes over time. Systematic errors are repeatable variations in the measurement as a result of the VNA test set. Typically, the systematic errors represent the larger contribution to measurement uncertainty. It is possible to mathematically reduce the systematic errors in a VNA measurement through calibration. Calibration comprises connecting a number of well-known standards to the VNA and measuring the response for each of the well-known standards. The systematic errors are quantified by calculating the difference between the measured response and the expected response for each of the well-known standards. This difference is used to develop an error correction model that may be used to mathematically remove systematic errors from a VNA measurement of the DUT. The calibration process effectively establishes a measurement reference plane at the point where the calibration standards are connected to the VNA measurement ports. Accordingly, it is possible to obtain a measurement of just the DUT by connecting the DUT to the measurement reference plane.
There are a number of calibration methods including a short-open-load-through (SOLT) calibration, a through-reflect-line (TRL) calibration, and an electronic calibration. A method and apparatus for an electronic calibration is disclosed in U.S. Pat. No. 5,434,511 entitled “Electronic Microwave Calibration Device” which issued on Jul. 18, 1995, the contents of which are hereby incorporated by reference herein. The SOLT, TRL and electronic calibration methods typically use calibration standards that are traceable to standards kept by the National Institute of Standards Technology (NIST). As one of ordinary skill in the art can appreciate, the quality of the error correction is directly related to the quality of the known standards. Most traceable calibration standards are made with coaxial connectors. As a practical reality, however, many devices either in a production or research environment do not have a coaxial connection. As a result, a series of standards disposed on a printed circuit board (PCB) and connected to an appropriate connector have been developed for use with non-coaxial measurement devices.
With specific reference to FIG. 1 of the drawings, there is shown a fixture 10 on which is disposed non-coaxial calibration standards and a measurement device path 104. The “on-board” calibration standards shown are the SOLT set of calibration standards and comprise an open circuit 20, a short circuit 30, a 50 ohm or 75 ohm load 40, and a through circuit path 102. Calibration of the fixture shown in FIG. 1 of the drawings comprises performing the conventional SOLT calibration process using the on-board calibration standards. Disadvantageously, the on-board calibration standards cannot be traced to an NIST standard. Specifically, the on-board short circuit 30 typically has some inductance associated with it. The on-board open circuit 20 typically has some capacitance associated with it and the load 40 does not typically represent a pure resistive 50 ohm or 75 ohm termination. It is difficult to accurately model the parasitic inductive and capacitive reactances that are associated with the on-board short and open, respectively. Because the calibration process does not have an accurate model of the parasitics associated with the calibration standards, there are irresolvable errors resulting from the calibration and error correction process. Measurement errors adversely affect the reliability of conclusions drawn from the measurements made. Accordingly, there is a need for an improved method and system for calibrating and measuring the high frequency characteristics of non-coaxial devices.